Support is requested for a molecular systematic investigation of the genus Caulerpa, a pantropical genus with ecological and biomedical importance. The specific objectives of the proposed study are to: 1) establish a robust molecular phylogeny of the Caulerpa based on four independent molecular data sets: nuclear ribosomal ITS sequences, 26S ribosomal DNA, and chloroplast encoded rbcL and psbA genes; 2) identify monophyletic groups for comparison with the current taxonomy of the genus, and characterization of morphological and biochemical evolution, 3) compare the pattern and rate of molecular evolution for different species, and 4) determine whether the inferred phylogenetic hypotheses are consistent with a presumed Indo-West Pacific origin of these algae, and if phylogenetic patterns of Caulerpa are correlated with the patterns of togther tropical marine organisms that may have shared a common marine biogeographic history. The large unicellular plants of Caulerpa are examples of single cells with a complex internal and external morphology with differentiated functions. Representatives of this genus have been recently implicated as a threat to biodiversity in the Mediterranean Sea and as the subject of toxicological studies on the secondary metabolite, caulerpenyne. A phylogenetic perspective of these algae will provide insights into the evolution and characterization of these features. Although phylogenetic and marine biogeographic hypothesis have been proposed for these algae, they have not been tested in the context of broad sampling of taxa with DNA sequence data. The focus of the project will include an evaluation of the evolution of morphological, ultrastructural and biochemical features, and elucidation of the rates and patterns of molecular variation at the with marine biogeographic hypotheses, (e.g., Tethyan vicariance models) to assess the historical causes of present day distribution. The PI has over fifteen years experience working with temperate and tropical marine algal taxonomy and ecology and another ten years working on the molecular systematics of tropical marine green algae. Successful completion of this project will increase our understanding of marine algal phylogeny, historical biogeography, and the rates of patterns of molecular evolution in marine organisms.